Inhibition of corrosion of metal surfaces by salt of phosphorodithioic acid



2,861,907 Patented Nov. 25, 1958 We t tes Pa Ofiice 2,861,907 INHIBITION OF CORROSION OF METAL SUR- IFQrICIIES BY SALT OF PHOSPHORODITHIOIC Thomas A. Butler, Cleveland, Ohio, assignor to The Lliliififll Corporation, Wickliife, Ohio, a corporation o No Drawing. Application September 28, 1956 Serial No. 612,597

Claims. (Cl. 148-6.17)

This invention relates to a process for the inhibition of corrosion of metal surfaces and in particular to a process which is particularly useful for the inhibition of corrosion of ferrous metal surfaces.

The corrosion of metal surfaces is of obvious economic significance in many industrial applications, and as a consequence the inhibition of such corrosion is a matter of prime consideration in such applications. It is of particular significance to users of steel and other ferrous alloys. metal alloys is largely in turn involves the metal to its oxide.

The theory which best explains such oxidation of ferrous metal surfaces postulates the essential presence both of water and oxygen. Even minute traces of moisture are sufficient, according to this theory, to induce the dissolution of iron therein until the water he comes saturated with iron ions. The presence of oxygen causes oxidation of the resulting ferrous hydroxide to ferric hydroxide which then settles out of solution and is ultimately convertedto ferric oxide or rust.

The above sequence of reactions can be prevented, or at least in large measure inhibited, by relatively impermeable coatings which have the effect of excluding either or'both moisture and oxygen from contact with the ferrous metal surfaces. Such coatings are, of course, subject to abrasion and other causes of physical deformation and to the extent that these coatings are penetrated or otherwise destroyed by such influences they are ineffective for the purpose desired. It is important that such coatings provide complete protection of all of the ferrous metal surfaces. If there is any portion of such a surface which is not so protected, regardless of how small the unprotected surface may be, the degree of protection afforded is considerably less than desirable. A satisfactory corrosion-inhibiting coating, then, must have the ability to resist substantial deformation upon impact, abrasion, etc. so that a uniform and complete film of protection is maintained in the face of such adverse influences.

It is accordingly a principal object of this invention to provide protection from corrosion for metal surfaces.

It is another object of this inventionto provide protection from the formation of rust on ferrous metal surfaces.

Other objects will be apparent from the ensuing description.

These objects are achieved by the process of inhibiting corrosion of metal surfaces which comprises applying to said surfaces a film of a polyvalent metal salt of a phosphorodithioic acid prepared by the process which comprises (a) The reaction of a mixture of a monohydric alcohol and from 0.25 to 4.0 equivalents of a polyhydric alcohol, with a phosphorus sulfide to form a phosphorodithioic acid, and

a matter of rust formation which over-all conversion of the free The corrosion of such ferrous fluidity on the one This formula, however, is not an entirely accurate representation of the phosphorodithioic acids of this invention because a substantial proportion of R and/or R is derived from a polyhydric alcohol. Consequently, R for example, may be attached not only to the oxygen of the above structural formula but also to the oxygen of a second phosphorodithioic acid nucleus, or in the case of a trihydric alcohol reactant, to the oxygen of a third phosphorodithioic acid nucleus. Or it may be that R, and R are the same alkylene radical, such alkylene radical being derived from a dihydric alcohol.

Inview of the above-described difficulties in characterizing the phosphorodithioic acids of this invention in terms of a chemical structure, it is deemed best to describe these acids in terms of the process by which they may be prepared and, accordingly, they are de scribed herein without reference to molecular structural formulas.

The monohydric alcohols contemplated as reactants in the process of this invention include principally the alkanols, although alcohols such as phenethyl and benzyl alcohols likewise may be used. For the most part, however, such alkanols as ethyl, butyl, hexyl, octyl, decyl, etc. alcohols are contemplated as the monohydric alcohol reactant. A particularly preferred subclass of mono: hydric alcohols are the octyl alcohols.

The polyhydric alcohol reactant may be selected from dihydric, trihydric and tetrahydric alcohols. Illustrative examples of such polyhydric alcohols include ethylene glycol, propylene glycol, butylene glycol, glycerol, pentaerythritol, butenediol, butynediol, etc. It is contemplated, likewise, to employ such polyhydric alcohols which may also contain aromatic substituents.

The proportions of monohydric and polyhydric alco-, hols which are to be used in the process of this invention are governed by considerations of solubility and hand, and of satisfactorily permanent film-forming properties on the other hand. It has been found that the use of a disproportionately large amount of polyhydric alcohol in the reaction mixture results in a product which is too insoluble and intractable to allow of convenient surface. If the proportion of polyhydric alcohol used in the reaction mixture is below a certain minimum quantity then the resulting product, although sufliciently fluid and soluble in many solvents, is not a satisfactory metallic surfaces do not afford a desirable degree of protection from corrosive influences. In view of these observations, it has been determined that the amount of polyhydric alcohol which can be used satisfactorily in the process here is within the range of 0.25 to 4.0 equivalents per equivalent of monohydric alcohol present in the reaction mass. It is particularly preferred to employ approximately equivalent amounts of monohydric and polyhydric alcohol reactants.

The term equivalent, consideration of the number of hydroxyl groups in the particular alcohol reactant. Thus the equivalent weight of ethylene glycol is 31, one-half of the molecular weight application to a metal as used above is based upon a 3 thereof. The equivalent weight of pentaerythritol is 34, one-fourth the molecular weight thereof. In view of the principal reaction of the process, i. e., phosphorus pentasulfide with the alcohols, it is evident that the relative quantities of alcohol reactants must in this case be discussed in terms of their equivalent weights.

The reaction by which phosphorodithioic acids are prepared is well known. It involves the reaction of one mole of phosphorus pentasulfide with four equivalents of an alcohol. 'In any discussion of the process of this invention, it is understood that this ratio of reactants is to be employed.

Although other phosphorus sulfides may be used, it is preferred both for reasons of economy and efficacy of reaction to use phosphorus pentasulfide.

The metal constituent of the corrosion-inhibiting agent of this invention is polyvalent. The divalent metals are preferred, and of these Zinc is particularly desirable. These metal salts are prepared from the phosphorodithioic acid, generally by neutralization thereof with the particular basic metal compound of the desired metal. Thus the prepartion of a zinc phosphorodithioate may be effected by neutralization of the phosphorodithioic acid with zinc oxide. Similarly, a barium salt may be prepared by neutralization with barium hydroxide octahydrate. Other methods of formation of such polyvalent metal salts may be accomplished by reaction of the phosphorodithioic acid with the free metal, such as metallic calcium.

The process by which these polyvalent metal salts are prepared is illustrated in detail by the following examples:

Example 1.To a solution of 1040 grams (8 equivalents) of n-octyl alcohol in 304 grams (8 equivalents) of 1,2-propylene glycol 2 hoursat 6070 C., 888 grams (4 moles) of phosphorus pentasulfide. The mixture was heated at 70-75 C. for 3 hours and filtered. A portion of the filtrate, weighing 678 grams (2 moles), was added slowly with stirring to a mixture of 89 grams (1.1 moles) of Zinc oxide and 741 grams of toluene. This mixture was filtered to yield a filtrate having the following analyses:

Percent P 5.0 Percent S 9.9 Percent 'Zn 4.8

Example 2.-A solution of 520 grams (4 equivalents) of isooctyl alcohol, 180 grams (4 equivalents) of 2- butent-1,4-diol in 1106 grams of toluene was prepared and treated with 444 grams (2 moles) of phosphorus pentasulfide. The resulting mixture was heated at 42-60 C for hours, then filtered. To 2000 grams (3.28 moles) of the filtrate there was added at 6065 C., 147 grams (180 moles) of Zinc oxide. The mixture was heated up to 115 C. over a 2-hour period by which time 31 "grams of water had been removed by distillation. The residue was filtered to yield a filtrate having the following analyses:

Percent P 5 .3 Percent 'S 10.7 Percent Zn 5.4

Example 3.To a solution of 382 grams (2.94 equivalents) of isooctyl alcohol and 131 grams (2.94 equivalents) of 2-butyne-1,4-diol in 789 grams of toluene there was added 326 grams (1.45 moles) of phosphorus pentasulfide. This mixture was heated at 7585 C. for 2 hours, then filtered. To 1150 grams (1.23 moles) of the filtrate there was added at 6065 C. 55 grams (0.68 mole) of zinc oxide. The mixture was heated at 115 C. for 2 'hours, removing 9 grams of water. The residue was filtered to yield a filtrate having the following analyses:

Percent P 3.6 Percent S 8.1. Percent Zn 3.6

there was added over a period of of dipropylene glycol and 1332 Example 4.-To a mixture of 650 grams (5.0 equivalents) of isooctyl alcohol and 1010 grams (15.0'equivalents) of dipropylene glycol there was added 1110 grams (5.0 moles) of phosphorus pentasulfide. The resulting mass was heated at 7080 C. for 2 hours and filtered. To a solution of 290 grams (1.0 mole) of the filtrate in 740 grams of isopropanol'there was added .5 grams (0.55 mole) of zinc oxide. This mixture was heated at 45 C. for 3.5 hours, then filtered. The filtrate showed the following analyses:

Percent P 3.6 Percent S 7.2 Percent Zn 2.8

Example 5 .-A mixture of 1560 grams (l2 equivalents) of isooctyl alcohol, 808 grams (12 equivalents) of dipropylene glycol and 1332 grams (6 moles) of phosphorus pentasulfide was heated at 65-70 C. for 4 hours and filtered. To 500 grams (1.58 moles) of 'the filtrate there was added 70.9 grams (0.87 'mole) of zinc oxide, and 550 grams of n-butyl alcohol. The mixture was stirred briefly and then filtered to yield a clear solution having the following analyses:

Percent P 4.5 Percent S 9.3 Percent Zn 4.5

Example 7.-To a mixture of 1116'grarns' (6 equivalents) of a mixture of C alcohols and 319 grams (6 equivalents) of diethylene glycol there was'added portionwise at 60-70 C. 666 grams (3 moles) of phosphorus pentasulfide. The resulting mixture was heated for'an additional 2 hours at 7075 C. and then filtered. A portion of the filtrate weighing 776 grams (2 moles) was added slowly to a suspension of 89 grams (1.1 moles) of zinc oxide in toluene. The mixture was heated at 40-60 C./25 mm. for 4 hours during which time 18 grams of water was collected. The residue was filtered and the filtrate had the following analyses:

Percent P 3.9 Percent S 7.6 Percent Zn 3.9

Example 8.-A mixture of 1560 grams (12 equivalents) of isooctyl alcohol, 808 grams (12 equivalents) grams (6 moles) of phosphorus pentasulfide was heated at C. for 3 hours and then filtered. To 584 grams (1 mole) of the filtrate dissolved in 640 grams of toluene there was added at C. 66 grams (1.18 moles) of iron filings. The

resulting mixture was heated at C. for 10 hoursand filtered. The filtrate was analyzed and found to have the following:

Percent P 4L9 PercentS 9.6 PercentFe 1.5

Example 9.A mixture of 975 grams (7.5 equivalents) of n'oct yl alcohol, 186 grams (6 equivalents.) .:of \dl.

ethylene glycol, 47 grams (1.5 equivalents) of glycerol and 833 grams (3.75 moles) of phosphorus pentasulfide was heated for 4 hours at 70-75 C. and then filtered. A portion of the filtrate weighing 722 grams (2 moles) was added portionwise to a suspension of 89 grams 1.1 moles) of zince oxide in 785 grams of toluene. The resulting mixture was stirred vigorously and then dehy-. drated by heating at 40-60 C./20 mm. for 2.5 hours. The residue was filtered; the filtrate showed the following analyses:

PercentP 4.8 Percent S 9.5 Percent Zn 4.9

following analyses:

Percent P 4.5 Percent S 8.5 Percent Zn 4.4

Example 11.A mixture of 1040 grams (8 equivalents) of isooctyl alcohol and 68 grams (2 equivalents) of pentaerythritol and 555 grams (2.5 moles) of phosphorus pentasulfide was heated at 75-102 C. for- 4 hours and then filtered. To a solution of 461 grams (3.94 moles) of the resulting phosphorodithioic acid in 1585 grams of toluene there was added portionwise 177 grams (2.18 moles) of zinc oxide. This mixture Was heated at 75-80 C. for 2 hours and then filtered. The filtrate was analyzed and showed the following:

Percent P 4.2 Percent S 7.7 Percent Zn 4.2

Example 12.-A mixture of 1560 grams (12 equivalents) of isooctyl alcohol, 373 grams 12 equivalents) of diethylene glycol and 1332 grams (6 moles) of phosphorus pentasulfide was heated at 50-70 C. for 2 hours and then filtered. A portion of this phosphorodithioic acid filtrate. weighing 1120 grams (4 moles) was heated at reflux temperature for 3 hours with 89 grams (4.94 equivalents) of water. A portion of the resulting product, a phosphoromonothioic acid, weighing 793 grams (3.0 moles) was added slowly and with stirring to a suspension of 128 grams (1.57 moles) of zinc oxide in 893 grams of toluene. This resulting mixture was heated at 50 C./20 mm. for 2 hours. Filtration of the residue yielded a toluene solution having the following analyses:

Percent P 4.8 Percent S- 6.5 Percent Zn 5.7

Example 13.-A mixture of 520 grams (10 equivalents) of neopentyl glycol (2,2-dimethylpropane-1,3-diol), 1300 grams (10 equivalents) of isooctyl alcohol and 1110 grams moles) of phosphorus pentasulfide was heated at 80-85 C. for 2 hours and filtered. A solution of 1734 grams (5.7 moles) of the filtrate in 1900 grams of toluene was treated portionwise at 75-80" C. and with stirring with 284 grams (3.97 moles) of zinc oxide. This mixture was heated for an additional 2 hours at 75 C. and again for another hour at 99-120 C. The mixture was then filtered to yield a toluene solution having the following analyses:

Percent P 4.6 Percent S 9.1 Percent Zn 5.4

Example 14.-To a mixture of 520 grams (4 equivalents) of isooctyl alcohol and 632 grams (4 equivalents) of 2,2-bis(4-beta-hydroxyethoxyphenyl)propane there was added 440 grams (2 moles) of phosphorus penta sulfide. This mixture was heated at 70-80 C. for 2 hours and filtered. A solution of 1156-grams (3 moles) of this filtrate in 600 grams of toluene was added to a suspension of 134 grams (1.65 moles) of zinc oxide in 2298 grams of toluene at 70-80 C. The mixture was heated at this temperature for an additional 3 hours and then filtered. The filtrate showed the following analyses:

Percent P 2.0 Percent S 3.9 Percent Zn 1.9

Example 15.-A phosphorodithioic acid was prepared at in Example 12 and 584 grams thereof (2 moles) was added to a suspension of 190 grams (1.1 moles) 'of barium hydroxide in toluene. The resulting mixture was heated at 55-60 C..for 2 hours and then 55-60 C./20 mm. The residue showed the following analyses:

Percent P 3.9 Percent S 7.5 Percent Ba 9.0

Example l6.-A phosphorodithioic acid was prepared as in Example 8 and to 500 grams (1.58 moles) thereof dissolved in 500 grams of isopropyl alcohol at 50-60" C. there was added 31.6 grams (0.79 mole) of calcium metal turnings. The resulting mixture was heated at reflux temperature for 6 hours and then filtered. The resulting solution showed the following analyses:

Percent P 5.0 Percent S---. 10.3 Percent Ca 2.8

Example 17.A mixture of 1040 grams (8 equivalents) of isooctyl alcohol, 304 grams (8 equivalents) of propylene glycol and 888 grams (4 moles) of phosphorus pentasulfide was heated at 70-75 C. for 3 hours and filtered. A mixture of 300 grams 1 mole) of this filtrate, 67.5 grams (0.58 mole) of cadmium oxide, gramsof benzene and 355 grams of isopropyl alcohol was heated at 40 C. for 2 hours, and then filtered. The filtrate showed the following analyses: 1

Percent P Q. 4.5

Percent S 9.2 Percent Cd 8.3

Example 18.-To a mixture of 118 grams (0.53 mole) of leadmonooxide, 201 gramsof benzene and 402 grams of isopropyl alcohol there was added 300 grams (1 mole) of the phosphorodithioic acid prepared as in Example .17. This mixture was heated at 35-45 C. for 3 hours and then filtered. The filtrate had the following analyses:

Percent P 3.9 Percent S 7.5 Percent Pb 10.9

The corrosion inhibiting compositions exemplified by the products of the above examples are usually applied to a metallic surface as a solution in an inert solvent. Such solvents ideally are suificiently volatile so as to allow their ready removal at convenient temperatures. Suitable solvents for this purpose include xylene, toluene, benzene, diethyl ether, alcohols, etc. In some instances the corrosion inhibitors may be applied as such, i. e., without a solvent, and in still others it is advantageous to apply them in aqueous emulsions. The method of application, liketoprovide protection agamst corrosion to all metal surfaces. Because of the pecullar susceptibility of iron to the moisture and oxygen content of theatmosphere, these compositions are especially valuable as protective films on ferrous metal surfaces.

The protection of ferrous metal surfacesfrom corrosion may be accomplished in part not only by protective films, but also by a'phenomenonknown as passivation. According to this latter method the'ferrous metal surface is'treatedwith a passivating agent such as nitric acid .or phosphoric acid. A brief reaction ensues after which the metal is then less susceptible to corrosion; it is passivated. A particularly useful method of passivating involves the treatment of a ferrous metal surface with a phcsphating solution. Such phosphatingsolutions impart a degree of .protection from corrosion by virtue of their ability to render a ferrous metal surface less susceptible to the formation of rust than it was previously.

It has been observed that an unusually high degree of protection from corrosion can be imparted to a ferrous metal surface'byutilizing a combination of such'a phosphating treatment together with the application of a film of the corrosion preventing composition of this invention. The net effect .of such a combination has been demonstrated to be considerably in excess of the additive effects of each of these measures when used alone.

The effectiveness of the polyvalent metal phosphordithioiates, described herein, as corrosion preventing agents for metal surfaces has been demonstrated by means of a Salt Fog Test. This test (ASTM (1954), Bll7-54T) involves the dipping of 3" x 3" steel panels into a solution of the sample to'be tested. The panels are withdrawn at a specified rate so as to insure a uniformly thick film. The solvent is allowed to'evaporate, and then 'the edges of the panels are painted with an air-drying enamel so as to assist in the reproducibility of the corrosion test to be carried out. Then the panels are placed in an enclosed cabinet which contains a5 percent aqueous solution of sodium chloride at95 C. This sodium chloride solution is agitated by means of a vigorous stream of air bubbled throughthesolution so as to produce a corrosiyensalt -steamatmosphereinthe cabinet. The resistance. of the, pan,els to corrosion is measured in terms of the time required to produce a single rust spot on the panel in this atmosphere. It is'a'pparent that the degree of protection from corrosion will be related to the thickness of the film which is deposited upon the metal surface, .so that any numerical measure of protection from'corrosion must take into account this factor of film thickness. Accordingly, the thickness of the filmis expressed in terms of milligrams (of film) per squarefoot (of metal .surface .area). The .use of this expression allows. an evaluation of the effectiveness. of the corrosion- 'preventing films of this invention in terms of hours.(of

rust-free exposure in a salt-steam-atrnosphere), Per. milligrarnsto'f film), persquare foot (of metal surface area) In orderto facilitate the comparison of these values for different films the salt-fog life" .described above .has

been expressed as hours/ O mg./ft.

.Theefiectiveness of the metal saltsof this invention as filmeforrning inhibitors ofcorrosion is shown by-the table below in which each salt-fog. life., represents an: average .of .results .from 1 six separate steel: panels containing films of 1 different. thicknesses.

1. 'Zinc salt of phosphorodithi ic acid prepared by reaction of- PrSr with i-CBHUOH and pentaerythritol (4:1 equivalentratio) 3. 1 2. 'Zinc salt; of phosphorodithi ic acid prepared by IQflOtlWl of P285 )with lorol 1 and diethylene glycol (1:1 equivalent 2 5 ratio 23. Zinc salt of phosphorodithl lc acid prepared by reaction of' P with H-CaHuOH and propylene glycol (1:1 equivalent ratio) 3:0 4. Zinc salt of phosphorodithioic acid prepared by reaction of P28 with II-CQHWOH ethylene glycol and glycerol (5:421 eguivalentratio) 3.9 5. Zinc salt of phosphorornonothioic acid prepared from i-CsHnOH and ethylene glycol (1: equivalent) 3.0 6. Uncoated steel panel 1. 0

1 Commercial mixture of C12 alcohols obtained by hydrogenation of coconut oil.

The data of the above table indicates the effectiveness of the film-forming compositions described herein. As indicated previously, however, the effectiveness of these compositions is even more striking when applied to metal surfaces which have previously been passivated. This is shown by data obtained by the treatment of phosphated ferrous surfaces with various compositions .of the type described herein. The method of phosphating the ferrous surfaces comprised immersing steel panels for 6 minutes in a solution containing 1.6 moles of phosphoric. acid (H PO.;), 0.4 mole of mono-ammonium phosphate (NH H PQ 1 mole ofzinc nitrate (Zn(NO and v1 mole of calcium nitrate (Ca(NO at -l80-l9 0 :F. The panelsare removed, rinsed with hot water, and then with dilute chromic acid (0.5 gram/liter) at 180.- l F. After drying the panels are ready to be coated with the corrosion-inhibiting compositions of this invention in the manner described previously. The results.of salt-fog life determinations of steel panels which have been phosphated and then coated with the polyvalent metal phosphorothioiates of this invention are shown in .the following table.

Table II Rust-proofing composition Product of Ex Product of Example 18 Edges otthc test specimens have beenpainted with an air-drying enamel.

Another principal application of the compositions of this invention depends'upon its utility as a primer for metal surfaces, particularlythose'which have beenphosphated or otherwisepre-treated. Asa primer-the metal salts of this invention not only afford a'high degree of protection from corrosion-but serve also to provide a relatively permanent bond between the metalsurface and the outside enamel coating.

Other. modes of applying the principle of the invention -may.rbe oemploycd, change being' ma dezasgregardsthe details .Edescribed,=, provided; the :features stated in any-of the following claims or the equivalent of such be employed.

I, therefore, particularly point out and distinctly claim as my invention:

1. The process of inhibiting corrosion of metal surfaces which comprises applying to said surfaces a film of a polyvalent metal salt of a phosphorodithioic acid prepared by the process which comprises (a) the reaction of a mixture of a monohydric alcohol and from 0.25 to 4.0 equivalents of a polyhydric alcohol, with a phosphorus sulfide to form a phosphorodithioic acid, and (b) conversion of said acid to a polyvalent metal salt thereof.

2. The process of claim 1 characterized further in that said metal surfaces are treated, prior to the application of said polyvalent metal salt, with a phosphating solution.

3. The process of claim 1 characterized further in that the monohydric alcohol of (a) is an octyl alcohol.

4. The process of claim 1 characterized further in that the monohydric alcohol of (a) is isooctyl alcohol.

5. The process of claim 1 characterized further in that the polyhydric alcohol of (a) is propylene glycol.

6. The process of claim 1 characterized further in that the polyhydric alcohol of (a) is dipropylene glycol.

7. The process of claim 1 characterized further in that the polyvalent metal salt is a zinc salt.

8. The process of inhibiting corrosion of metal surfaces which comprises applying to said surfaces a film of a zinc salt of a phosphorodithioic acid prepared by the process which comprises (a) the reaction of a mixture of a monohydric alcohol and an approximately equivalent amount of polyhydric alcohol, with phosphorus pentasulfide to form a phosphorodithioic acid and (11) conversion of said phosphorodithioic acid to the zinc salt thereof by reaction with zinc oxide.

9. The process of claim 8 characterized further in that the metal surfaces are ferrous metal surfaces.

10. The process of claim 9 characterized further in that said ferrous metal surfaces are treated, prior to the application of said zinc salt, with a phosphating solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,063,629 Salzberg Dec. 8, 1936 2,564,864 Thompson Aug. 21, 1951 2,671,758 Vinograd et al. Mar. 9, 1954 

1. THE PROCESS OF INHIBITING CORROSION OF METAL SURFACES WHICH COMPRISES APPLYING TO SAID SURFACES A FILM OF A POLYVALENT METAL SALT OF A PHOSPHORODITHIOIC ACID PREPARED BY THE PROCESS WHICH COMPRISES (A) THE REACTION OF A MIXTURE OF A MONOHYDRIC ALCOHOL AND FROM 0.25 TO 4.0 EQUIVALENTS OF A POLYHYDRIC ALCOHOL, WITH A PHOSPHORUS SULFIDE TO FORM A PHOSPHORODITHIOIC ACID, AND (B) CONVERSION OF SAID ACID TO A POLYVALENT METAL SALT THEREOF.
 2. THE PROCESS OF CLAIM 1 CHARACTERIZED FURTHER IN THAT SAID METAL SURFACES ARE TREATED, PRIOR TO THE APPLICATION OF SAID POLYVALENT METAL SALT, WITH A PHOSPHATING SOLUTION. 